The Spotlight index is a few gigabytes, and rebuilding it costs a day
Every volume carries a hidden index proportional to its contents. When it is worth rebuilding, when to exclude a folder, and what it costs you.
Every volume on a Mac carries a hidden .Spotlight-V100 folder holding the search index for its contents. It is proportional to how many files there are rather than how large they are, so a drive full of source code has a bigger index than one holding a few large videos.
Looking at it
sudo du -sh /.Spotlight-V100 2>/dev/null
sudo du -sh /Volumes/*/.Spotlight-V100 2>/dev/nullA couple of gigabytes on a well used internal drive is normal. What is not normal is an index still growing days after a large copy, which usually means indexing is stuck rather than that the index is large.
Whether indexing is actually running
mdutil -s /
sudo mdutil -s /Volumes/*That reports whether indexing is enabled and complete per volume. If a fan has been running for two days and a process called mdworker is busy, this is the command that tells you whether it is progress or a loop.
Rebuilding, and what it costs
sudo mdutil -E /That erases and rebuilds the index for a volume. It is the standard fix for search returning nothing or returning nonsense, and the cost is real: until it finishes, search is incomplete, and on a large drive it takes hours during which the Mac is busier and warmer than usual. Do it when you can leave the machine alone, not before a flight.
Turning it off, which is usually the wrong trade
You can disable indexing per volume, and the index goes with it. On an internal drive this breaks Spotlight search, Siri suggestions and every app that uses metadata search, which is most of them. It is a reasonable choice on an external archive drive you never search and never for the startup disk.
sudo mdutil -i off /Volumes/ArchiveDriveA softer option is to exclude specific folders in System Settings, Siri and Spotlight, Search Privacy. Excluding a folder of build output or virtual machines keeps the index smaller and makes search results better, since those folders were never useful matches anyway.
Where it fits
The index is a few gigabytes and it earns them. It is worth knowing about mainly because it appears in the System Data bucket unexplained, and because it is one of the reasons a drive reports more used space than the files you copied.
Common questions
How much space does the Spotlight index use?
Usually one to a few gigabytes on a well used internal drive, in proportion to the number of files rather than their size. A drive with many small files, such as source code, carries a larger index than one holding a few large videos.
How do I rebuild the Spotlight index on a Mac?
Run sudo mdutil -E / for the startup volume. Search is incomplete until the rebuild finishes, which takes hours on a large drive and keeps the Mac busy, so start it when you do not need the machine.
Can I delete the .Spotlight-V100 folder?
Not by hand. Use mdutil, which stops the indexing service before removing the index. Deleting the folder while indexing is running can leave the volume stuck reindexing indefinitely.
Should I turn off Spotlight indexing to save space?
On the startup disk, no. It breaks search across the system and saves only a few gigabytes. On an external archive drive you never search it is reasonable, and excluding specific folders in Search Privacy is the better middle ground.